A wide variety of current diagnostic and other biochemical tests employ a reagent which undergoes an eye detectable colour change in the presence of the analyte. The reagent is often conveniently carried on a test strip. Optics may be provided to assist the comparison of the observed colour change with a standard colour chart. Alternatively, the optical absorption may be measured at one or more selected wavelengths Whilst some arrangements permit the measurement of absorption through detection of transmitted light, reflectrometric arrangements are usually more convenient.
In many fields, the amount of sample available for analysis is very much restricted. It is accordingly desirable to have apparatus which is capable of analysing small drops or thin layers of sample. In many cases, immunoassay techniques being good examples, the thin layer approach has important further advantages. It becomes possible, for example, to immobilise the reagent (whether antibody or antigen) upon a surface and thus remove the need for a separation step. Examples of related techniques are enzyme-linked colorimetric assays and tests for other biochemical analytes.
In the particular field of immunoassays, the technique of internal reflection spectroscopy is known. A thin layer of sample material is established on a surface of a transparent optical element and measurements made of internal reflection of light at the interface with the sample. The same technique can be employed to measure fluorescence within the thin layer sample and other techniques have been suggested for detecting a shift in refractive index or polarisation.
A drawback of the techniques proposed hitherto is that they rely upon optical detection of reflected or transmitted light with subsequent signal processing providing the required measure of absorption, fluorescence or other optical parameter of interest. This is believed to have hindered the development of apparatus to meet the demands of specific applications. Thus, it is desirable in many applications to produce apparatus which is both compact and rugged. Under certain circumstances, where for example the analyte is potentially toxic or poses a microbiological contamination or health risk, it is desirable to have analytical apparatus of a form which is disposable after each analysis. In some cases, it would be advantageous to permit a number of analytical procedures to be conducted upon the same body of sample. The techniques employed hitherto do not lend themselves to embodiment in these forms.